1. Field of the Invention
This invention relates to an electronically controlled anti-lock braking system (ABS) for a motor vehicle.
2. Description of Related Art
Electronically controlled ABS systems are known in which a microprocessor is used to control release of the brakes of a motor vehicle in response to a determination by the microprocessor that the brakes have locked or are approaching a locked up condition. When such a determination is made, a signal from the controller typically actuates an electro-magnetic brake release, e.g., a solenoid controlled valve in the brake system.
A determination of wheel locking generally requires measurement of actual wheel speed or deceleration, and a determination that the actual wheel speed or deceleration is respectively lower than or greater than a predetermined wheel speed or deceleration limit indicative of an incipient wheel-lock situation. It is also possible to make the determination based on a measurement of actual G-force and comparison with a wheel-speed derived G-force curve.
Such systems are all subject to the common problems of spurious determinations resulting from controller malfunction, and malfunctions in the electro-magnetic brake release. Improper functioning of the ABS can, of course, lead to disastrous consequences, either from failure to release the brakes when the wheels are locked, or from an unintended brake release when braking is required.
As a result, it has been proposed to provide fail-safe circuits for the main controller, and to provide feedback from the brake release to the controller in order to permit the controller to monitor the brake release. Such proposals have not proven to be adequate, given the life-threatening potential of even one malfunction.
A typical "fail-safe" ABS is shown in U.S. Pat. No. 4,700,304, to Byrne et al. In Byrne et al, an analog fail-safe circuit is provided which includes a fuse in series with the drive circuits of an electro-magnetic brake release. When the fuse is blown, the brake release is prevented from operating.
The central ABS controller is a microprocessor which sends out periodic pulses to the fail-safe circuit which disables the fail-safe means from blowing the fuse. The fail-safe circuit of Byrne et al. cannot act independently of the microprocessor, being completely dependent on proper output by the microprocessor of the periodic pulses. If no pulse is received within a predetermined period of time, the fuse automatically blows, but as long as the pulses are received, the fail-safe means is disabled.
An analog system of the type shown in Byrne et al., in addition to being subject to such microprocessor internal errors as, for example, misreading of a brake-release feedback signal, suffers from an unacceptably slow response time. As a result, it has been proposed to substitute a microprocessor for the conventional analog fail-safe circuit.
An example of a system which uses separate microprocessors for detecting malfunctions is disclosed in U.S. Pat. No. 4,709,341 to Matsuda. In Matsuda, identical microprocessors are provided which monitor each other in addition to performing as ABS controllers for respective wheels of the motor vehicle. Due to costs, such a system is suitable only where separate controllers are required, and in addition suffers from the drawback that it is subject to systematic errors from such sources as radio frequency interference and power supply fluctuations which affect all of the identical microprocessors of Matsuda in exactly the same way due to their identical structure and functions.